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The geology of the Clermont 4-mile Sheet area, Queensland

Abstract

Parts of three major structural units are exposed in the Clermont 4-mile Sheet area: the Anakie Inlier of pre-Devonian Anakie
Metamorphics; the Drummond Basin of Lower Carboniferous Drummond Beds; and the Bowen Basin of Permian Collinsville Coal Measures,
Middle Bowen Beds and Passage Beds, Upper Bowen Coal Measures, and Triassic Carborough Sandstone. Lying between the Anakie
Inlier and the Bowen Basin are the Upper Devonian to Lower Carboniferous Bulgonunna Volcanics. The Anakie Metamorphics are
an unknown thickness of folded and sheared quartz schist, quartz mica schist, and mica schist that are older than Devonian.
South of Clermont township, they are overlain unconformably by a sequence of Middle Devonian coralline limestone (the Douglas
Creek Limestone), brachiopod-bearing siltstone, and Theresa Creek Volcanics that include flows of andesite and trachyandesite
and equivalent tuffs. In the west, the Anakie Inlier is overlain unconformably by 14,000 to 20,000 feet of Lower Carboniferous
Drummond Beds of rhyolite, conglomerate, and plant-bearing sandstone and siltstone, which are folded into a broad syncline.
In the east, the Anakie Inlier is overlain unconformably by an estimated 16,000 feet of Upper Devonian to Lower Carboniferous
Bulgonunna Volcanics of acid volcanics and plant-bearing siltstone. At Clermont, the Anakie Metamorphics are overlain unconformably
by Permian marine and 'freshwater rocks, and at Blair Athol by Permian coal measures. The Permian rocks of the Bowen Basin
overlie unconformably the Bulgonunna Volcanics. They comprise, from the base upwards, at least 150 feet of Collinsville Coal
Measures, including at least 30 feet of coal, 1000 to 1500 feet of Middle Bowen Beds .(marine quartz greywacke and siltstone,
with two coquinites near the exposed base) that inter finger with an estimated 2000 feet of Passage Beds (alternating marine
and freshwater quartz greywacke and siltstone with coal seams), and an estimated 3,500 feet of Upper Bowen Coal Measures
(freshwater quartz greywacke, sandstone, lithic sandstone, and coal seams). The Upper Bowen Coal Measures are overlain, probably
conformably, by 850 feet of Triassic Carborough Sandstone. The Permian Blair Athol Coal Measures, which comprise 800 feet
of conglomerate, sandstone, siltstone, and shale and several coal seams, including one a hundred feet thick, overlie unconformably
the Anakie Metamorphics in an isolated basin. Igneous activity was widespread in the area. The Theresa Creek Volcanics were
intruded, probably in the Carboniferous, by monzonite; the Permian rocks in the north-eastern part of the Sheet area by syenite
and granodiorite; and the Bulgonunna Volcanics and Permian rocks in the Peak Range and elsewhere by the Tertiary Peak Range
Volcanics and by dykes of basalt and trachyte. Tertiary olivine basalt was extruded over a wide area. The Peak Range Volcanics
are an exceptionally well-exposed and well-preserved suite of hypabyssal intrusions of soda-rich rhyolite and trachyte. The
Permian rocks are gently folded except where they are arched up by igneous intrusion. Broad folds in the Cherwell Range and
small domes elsewhere are potential reservoirs for petroleum generated in bodies of dark siltstone.